What is the ruling on delaying leaving the washroom in order to cleanse oneself from impurity by inserting a finger with water, and is this action considered a sin, and what is the legitimate Islamic advice for getting rid of this suffering?

This act is a result of obsessive whisperings (waswas), and it is advised to turn away from them and not pay them any attention. In istinja' (cleaning oneself after defecation or urination), it is sufficient to have a strong presumption that purification has occurred. So, perform istinja' until you strongly presume the area is clean, then stand up immediately and do not pay attention to the whisperings. However, if this act is for pleasure, then it is a blameworthy and reprehensible matter. You must fear Allah, repent to Him, and strive against your soul, for whoever strives against his soul, Allah will help him and grant him success. "And those who strive for Us – We will surely guide them to Our ways."